1. Field of the Invention
The present invention relates to a method and an apparatus for processing a color video signal.
2. Description of the Prior Art
It has been known heretofore that, in computer graphics for example, a capability of displaying 256 (=2.sup.8) colors per pixel is generally required. In other words, that requirement is satisfied by allocating 8 bits to each pixel.
Recently, however, there are cases in which another requirement exists for the processing or displaying of a natural picture such as an image picked up by a video camera. In such a case, the conventional 256-color display capability is not sufficient.
In the case of a computer graphics system equipped with a display function of 2048 pixels.times.2048 pixels, if 8 bits are allocated to each of the red, green and blue colors in one pixel to increase the color display capability per pixel, it follows that a total of 24 bits are needed for each pixel, and therefore the total of the bits required for a complete image or frame is ##EQU1## Thus, the display memory required has become excessive in size. In addition, an extended time period is necessary for processing the image. Even if a hard disk drive is employed only a few frames of images can be stored. As a result, satisfactory adaptability for practical use is unattainable.
As a solution for the above problems, there is a known method of extending the color display capability by means of a color look-up table while still allocating 8 bits per pixel. An example of such a color look-up table might consist of memories having 4 addresses each composed of 8 bits and designated by 2-bit red and blue address signals respectively, and also 16 addresses designated by 4-bit green address signals, with 8-bit color saturation data being stored in each address. A total of 24 such addresses are, designated by 8-bit color look-up table address signals stored in the addresses corresponding to the individual pixels in the display memory. Accordingly, although merely 256 addresses are selectable in the color look-up table, a total of 16,777,216 values can be derived since the output of the color look-up table is composed of 24 bits in all. Therefore, despite the limited simultaneous display of merely 256 colors in one frame, it is rendered possible to display a total of 16,777,216 colors by rewriting the contents of the color look-up table during a blanking interval or the like. Consequently, due to the use of such a color look-up table, many hues and gradations (color saturations or values of color components) are representable, even with limited display memory capacity, resulting in the display of a colorful image.
However, even in the aforementioned technique, the kinds of colors reproduced may be insufficient to bring about naturalness with respect to the luster and tint. For example, in displaying lustrous fruits, merely 2 bits are allocated to red and blue respectively and, due to there being only four gradations, quantization is rendered conspicuous with color changes appearing like stripes.
In addition to the above, since merely 8 bits/3.congruent.2.7 bits on average are allocated with respect to each of red, green and blue, there arises a problem of insufficiency in the gradations with further disadvantages including reduction of the resolution and deterioration of the picture quality.